Embodiments of the present invention generally relate to measuring cardiac impedance, and more particularly to methods, devices and systems that improve cardiac impedance measurement signal quality in the presence of pacing pulses.
Implantable medical devices (IMD) are well known in the art. The IMD may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation. The IMD may also take the form of implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable medical devices may also incorporate more than one of a pacemaker, a cardioverter and a defibrillator. Defibrillators may include “shock only” functionality or, in addition to shocking functionality, a defibrillator may be capable of providing cardiac resynchronization therapy (CRT) functionality.
IMDs are coupled to one or more leads that include electrodes to sense one or more types of information and to deliver various types of therapy. The IMDs typically include various sensing circuitry and logic that monitor a heart for cardiac signals, and analyzes the cardiac signals to identify normal sinus rhythm, arrhythmias and the like. The sensing circuits sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P-waves) and intrinsic ventricular events (R-waves). By monitoring P-waves and/or R-waves, the IMD circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
IMDs also include sensing circuitry and logic that utilize impedance cardiograph, such as for the purpose of monitor hemodynamic output. Intracardiac impedance recordings, such as using trans-venously implanted leads, are now being used in implantable devices. The IMD collects impedance data in various manners. For example, one approach is to deliver small short current bursts between two electrodes proximate to the heart and simultaneously measure a voltage potential between the same electrodes or different electrodes. The current source and voltage potential are used to derive impedance data. The impedance data is collected over one or more cardiac cycles to monitor a hemodynamic output of the heart.
However, certain limitations exist today in connection with collecting impedance data, given that the IMD collects impedance data, while the same IMD performs normal sensing and pacing operations. The pacing pulses from the IMD (e.g., pacemakers, ICD, CRT, etc.) may interfere with the recording of impedance data. The effect on the impedance data may include crosstalk from the pacing pulse or loss of impedance measurement due to disconnection of the impedance sensing circuitry during pacing pulse delivery. In general, the change of intracardiac impedance (e.g., the maximum and minimum dynamic range of impedance data recorded per cardiac cycle) varies in the order of 0.2 ohms to 1.5 ohms between peak and valley measurements. Given such a small dynamic range, even small artifacts, such as related to hardware crosstalk or post-pace blanking, discharge and recharge, fall within the impedance range and can impact the fidelity, signal morphology and hence the IMD's utility as a sensor to derive hemodynamic information.